1. Field of the Invention
The present invention relates to an optical amplifying apparatus which is used in an optical communication system or the like and, more particularly, to an optical amplifying apparatus and an optical amplifying method which can amplify light at a predetermined level, even when a predetermined optical signal is rejected from a wavelength-division multiplexed optical signal in an optical repeater station of the optical communication system. Further, it relates to an optical communication system utilizing the optical amplifying apparatus.
2. Description of the Related Art
FIG. 16 is a view showing the structure of a conventional optical communication system.
As in FIG. 16, the optical communication system is structured by including an optical transmitting station 501 which generates a WDM optical signal in which a plurality of optical signals in the number of m with different wavelengths from each other are wavelength-multiplexed, an optical transmission line 502 through which the WDM optical signal outputted from the optical transmitting station 501 is transmitted, and an optical receiving station 503 into which the transmitted WDM optical signal is inputted to be received and processed. Moreover, in the optical communication system, optical repeater stations 504 are connected in the optical transmission line 502. A plurality of the optical repeater stations 504 are provided in the optical transmission line 502 as necessary, and each of the optical repeater stations 504 may include an optical amplifier 531 which amplifies the WDM optical signal to a predetermined optical level in order to compensate transmission loss occurring in the optical transmission line 502, or it may include an optical add/drop multiplexer (hereinafter abbreviated to xe2x80x9cOADMxe2x80x9d) 532 for dropping/adding an optical signal corresponding to a predetermined channel (hereinafter abbreviated to xe2x80x9cch.xe2x80x9d) from/to the WDM optical signal.
The optical transmitting station 501 is structured by including, for example, a plurality of optical senders (hereinafter abbreviated to xe2x80x9cOSxe2x80x9d) 511-1 to 511-m in the number of m, each of which generates an optical signal corresponding to the respective channels of the WDM optical signal, an optical multiplexer (hereinafter abbreviated to xe2x80x9cMUXxe2x80x9d) 512 which multiplexes wavelengths of the respective optical signals outputted from the OSs 511-1 to 511-m, and an optical amplifier 513 which amplifies the WDM optical signal outputted from the MUX 512.
The optical receiving station 503 is structured by including, for example, an optical amplifier 521, an optical demultiplexer (hereinafter abbreviated to xe2x80x9cDEMUXxe2x80x9d) 522 and optical receivers (hereinafter abbreviated to xe2x80x9cORxe2x80x9d) 523-1 to 523-m. The WDM optical signal which is inputted from the optical transmission line 502 into the optical amplifier 521 is amplified therein and outputted to the DEMUX 522, in which its wavelength is demultiplexed to each of the optical signals corresponding to the respective channels. The demultiplexed optical signals of the respective channels are inputted into the ORs 523-1 to 523-m, respectively, each of which is structured by including a photo diode, a demodulator and so on, to be received and processed therein.
The optical amplifying apparatus 531 of the optical repeater station 504 is structured by including a first optical fiber amplifier doped with an rare earth element, an optical attenuator, and a second optical fiber amplifier doped with an rare earth element. The rare earth element is selected corresponding to an amplification wavelength band as necessary and, for example, an erbium element (elemental symbol: Er) is used in amplifying a 1550 nm band. The optical fiber amplifier is designed to obtain an appropriate gain with a predetermined wavelength multiplexing number so that a gain deviation becomes 0, and it is controlled to obtain the constant gain (constant gain control). The optical attenuator controls the optical amplifying apparatus 531 to control its output constantly.
The OADM 532 of the optical repeater station 504 is structured by including, for example, an optical coupler, an optical filter and an optical multiplexer/demultiplexer. The inputted WDM optical signal is divided into two in the optical coupler, and one of these is inputted into the optical filter and the other is used for receiving/processing a predetermined optical signal corresponding to a channel to be dropped in the OADM. The optical filter filters the inputted WDM optical signal to reject the predetermined optical signal. The optical multiplexer/demultiplexer multiplexes wavelengths of the WDM optical signal from which the predetermined optical signal is rejected and an optical signal to be newly added in the optical repeater station 504.
How the WDM optical signal is transmitted in the optical communication system like the above is explained as follows. When, for example, m is 4, that is, when a 4-wave WDM optical signal is transmitted, it is generated in the optical transmitting station 501 and repeated/amplified in an optical repeater station 504-1, an optical signal corresponding to, for example, ch. 3 is dropped/added therefrom/thereto in an optical repeater station 504-2, and it is repeated/amplified in an optical repeater station 504-3. Thus, it is repeated/amplified and dropped/added in the optical repeater stations 504 in sequence, to be received in the optical receiving station 503. In this case, a cutoff wavelength of the optical filter of the OADM 532-1 is set so as to filter the ch. 3.
FIG. 17 are views showing states of the 4-wave wavelength-division multiplexed optical signal being amplified.
FIG. 17A shows a state of the 4-wave WDM optical signal after being amplified in the optical repeater station 504-1, FIG. 17B shows a state in which the optical signal corresponding to the ch. 3 is dropped therefrom and then newly added thereto in the optical repeater station 504-2, and FIG. 17C shows a state of the WDM optical signal, to which the ch. 3 is added/dropped thereto/therefrom, after being amplified in the optical repeater station 504-3. Lateral axes of FIG. 17 show a wavelength (ch.) and vertical axes show an optical level.
As shown in FIG. 17B, when the predetermined optical signal (ch. 3, in FIG. 17B) is dropped/added from/to the WDM optical signal, the optical filter of the OADM 532 rejects the predetermined optical signal including ASE. Therefore, as shown in FIG. 17C, the WDM optical signal after the predetermined optical signal is dropped/added therefrom/thereto has the different optical levels of the ASE between the respective optical signals, after being amplified in the optical repeater station 504.
Supposing that, for example, the 4-wave WDM optical signal is amplified in an optical repeater station 1, the ch. 3 is dropped/added therefrom/thereto in an optical repeater station 2 and it is amplified in an optical repeater station 3. As to an output from the optical repeater station 3, as shown in FIG. 17C, the ASEs of ch. 1, ch. 2 and ch. 4 have a noise level from the two optical repeater stations, but the ASE of the ch. 3 has a noise level from one optical repeater station. When the ch. 3 is dropped/added therefrom/thereto during the transmission, optical levels of the ASEs differ between the ch. 1, ch. 2, ch. 4 and the ch. 3.
It should be mentioned that the optical amplifying apparatuses 513, 531, 521 which are provided in the optical transmitting station 501, optical repeater station 504 and the optical receiving station 503 are normally controlled so that outputs from the optical amplifying apparatuses become constant (output constant control). This is because an input optical level of the optical transmission line 502 is limited in order to prevent nonlinear optical effects such as a self-phase modulation (SPM), a cross-phase modulation (XPM) and the like from occurring in the optical transmission line 502.
As shown in FIG. 17A, since amplified spontaneous emission (ASE) is usually caused in the optical amplifying apparatus, the ASE is added to the output from the optical amplifying apparatus. When its output is constantly controlled so that an output per wave becomes P0, the following formulas hold therefore:                                                                         P                Tout                            =                              xe2x80x83                            ⁢                              m                xc3x97                                  P                  0                                                                                                        =                              xe2x80x83                            ⁢                                                                    P                    Tin                                    xc3x97                  G                                +                                  2                  ⁢                                      n                    sp                                    ⁢                  h                  ⁢                                      xe2x80x83                                    ⁢                  ν                  ⁢                                      xe2x80x83                                    ⁢                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  f                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          G                      -                      1                                        )                                                                                                          (                  Formula          ⁢                      xe2x80x83                    ⁢          1                )            xe2x80x83xcex94P0={2nsphxcexdxcex94f(Gxe2x88x921)}/mxe2x80x83xe2x80x83(Formula 2)
Incidentally, m is a multiplexing number of the wavelength-division multiplexed optical signal (wavelength number, channel number), PTout is a total output of the optical amplifying apparatus, PTin is a total input of the optical amplifying apparatus, nsp is a spontaneous emission coefficient of the optical amplifying apparatus, G is a gain of the optical amplifying apparatus, hxcexd is energy (j) of a photon, and xcex94f is a bandwidth of the optical amplifying apparatus.
Since the total output power is controlled fixedly according to the output constant control, a second term of the formula 1 is an error component as shown in the formula 1, and hence the optical level of the optical signal per wave (per channel) becomes smaller than the predetermined optical level P0 by the xcex94P0.
For this reason, in the conventional art, the optical amplifying apparatuses 513, 531, 521 correct the xcex94P0 and constantly control the outputs so that the optical levels of the optical signals from the optical amplifying apparatuses 513, 531, 521 become P0.
By thus correcting the xcex94P0, an optical signal-to-noise ratio (hereinafter abbreviated to xe2x80x9coptical SNRxe2x80x9d) after a multi-repeating is improved.
Here, the optical SNR is expressed as:
1/Optical SNR=1/Optical SNR(1)+1/Optical SNR(2)+ . . . +1/Optical SNR(n)xe2x80x83xe2x80x83(Formula 3) 
Optical SNR(j)=Pinj/(2nspjhxcexdxcex94f)xe2x80x83xe2x80x83(Formula 4) 
Incidentally, the optical SNR is an optical SNR after passing an nth optical amplifying apparatus, the optical SNR (j) is an optical SNR when only a jth optical amplifying apparatus is used, Pinj is optical power of the input light of the jth optical amplifying apparatus, and nspj is ASE in the jth optical amplifying apparatus.
FIG. 18 are views showing the optical SNR and a level diagram with/without output correction.
FIG. 18A is a view showing a difference between the optical SNR with the output correction and without the output correction, in which ▪ shows the case with the output correction and ♦ shows the case without the output correction. A lateral axis shows a number of the optical repeater station (number of the optical amplifying apparatus) and a vertical axis shows the optical SNR shown in dB. FIG. 18B is a view showing a difference between the level diagram with the output correction and without the output correction, in which ▴ shows an optical level of the optical signal with the output correction, xc3x97 shows an optical level of the optical signal with the output correction and with the ASE being added thereto, ♦ shows an optical level of the optical signal without the output correction, and xe2x97xaf shows an optical level of the optical signal without the output correction and with the ASE being added thereto. A lateral axis shows a number of the optical repeater station (number of the optical amplifying apparatus), and a vertical axis shows a level of average optical power of the channel shown in dBm/ch.
As conditions of a simulation, gain deviations of the optical amplifying apparatuses 513, 531, 521, a deviation of the optical transmission line 502 and the like are set as 0, the output optical levels of the optical amplifying apparatuses 513, 531, 521 without the output correction are set as 0 dBm/ch., transmission loss of the optical transmission line 502 is set as 25 dB, a noise figure (hereinafter abbreviated to xe2x80x9cNFxe2x80x9d) is set as 7 dB, and an amplification bandwidth is set as 30 nm. Further, the optical repeater station 404 does not include the OADM 532.
As shown in FIG. 18, transmission characteristics are improved with the output correction for compensating the ASE, compared with the case without the output correction. However, in the output correction, the ASE is assumed to be uniformly added to the respective optical signals of the WDM optical signal, and the level of the average optical power of the channel increases as it transmits through the optical repeater stations.
As described above, the optical filter of the OADM rejects the predetermined optical signal including the ASE. Hence, when amplified in the optical amplifying apparatuses in the optical repeater stations, the WDM optical signal after the predetermined optical signal is dropped/added therefrom/thereto is not uniform and its ASE level differs between the respective optical signals. Therefore, the suitable output correction is difficult to be achieved in the conventional art.
Moreover, since the optical fiber amplifier of the conventional optical amplifying apparatus is designed to obtain the appropriate gain with the predetermined wavelength multiplexing number and is controlled to obtain the constant gain, the gain deviation is caused when the multiplexing number changes in the OADM because the appropriate gain is not obtained.
It is an object of the present invention to provide the optical amplifying apparatus and the optical amplifying method which correct the output with consideration of the optical level of the ASE by each optical signal.
It is another object of the present invention to provide the optical communication system utilizing the optical amplifying apparatus.
The above objects are achieved by an optical amplifying apparatus comprising a correction part for correcting a predetermined fixed value of an output optical level by a correction amount according to an optical power of amplified spontaneous emission. The correction amount is computed by, for example, predetermined formulas. Further, the correction amount is obtained by adjusting both or one of a gain of the first optical amplifying part and a gain of the second optical amplifying part in the optical amplifying apparatus which comprises, for example, an optical attenuating part provided between a first optical amplifying part and a second optical amplifying part, for attenuating output light of the first optical amplifying part, and a control part for adjusting the output optical level to the predetermined fixed value.
Moreover, the above objects are achieved by an optical communication system comprising an optical transmitting station, an optical repeater station, an optical receiving station, and an optical transmission line for connecting the respective stations, and wherein the optical repeater station comprises the above correction part.
According to the present invention, when a WDM optical signal is optically amplified, the output is fixedly amplified for each channel by the correction amount which is a value obtained with the ASE levels taken into consideration, so that a fluctuation in the level of average optical power of the channels is suppressed even if an optical signal is dropped/added therefrom/thereto during repeated transmissions. Therefore, the optical communication system according to the present invention realizes suppression in gain deviation among the optical signals which correspond to each of the channels of the WDM optical signal. According to the optical communication system in the present invention, it is possible to prevent an optical SNR from degrading due to a decrease in the number of channels even when a number of times an optical signal is wavelength-multiplexed is small.